Field of the Invention
This invention relates to a storage system, a control program for a storage system, and a controller. More specifically, this invention relates to a storage system which has a first and a second fixed storage device, a control program for the storage system, and a controller.
Description of the Related Art
According to improvement of high resolution of video data and image data, and networks proliferation etc., it is required to increase largely recording capacity for fixed storage devices such as a HDD (Hard Disk Drive), a SSD (Solid State Drive), or the like. It is required for a storage system with a fixed storage device, to read/write data fast (improve performance) and store data safely (improve the backup function).
In case that a storage system stores all the data in one fixed storage device, all the data may be lost by the occurrence of trouble in the fixed storage device. In particular, trouble can easily arise in HDDs, since HDDs have movable parts.
There are technologies of RAID (Redundant Arrays of Inexpensive Disks) using a plurality of fixed storage devices, to improve performance and the backup function of a storage system. RAID has a plurality of writing methods such as RAID0, RAID1, or the like.
RAID0 is a technique which is so called a striping. When data writing by RAID0, data is divided into a plurality of divided data pieces by the block. Each of a plurality of divided data pieces is written in the corresponding fixed storage device. When data reading by RAID0, the divided data pieces are read from each of a plurality of fixed storage devices in parallel. Herewith, writing speed for the fixed storage device can be improved, and the performance of the storage system can be improved.
RAID1 is a technique which is so called mirroring. When data writing by RAID1, data is copied. Each of same data pieces are written in a plurality of fixed storage devices. When data reading by RAID1, data pieces are read from only one normal fixed storage device, out of the plurality of fixed storage devices. Herewith, when trouble occurs at a fixed storage device out of a plurality of fixed storage devices, the data can be read from a normal fixed storage device. Therefore, the backup function of the storage system can be improved.
Documents 1 to 3 below disclose techniques relating to the RAID. Document 1 below discloses a storage control method of a storage device which has a plurality of HDDs. According to the storage control method, the storage device stores data by using two storage methods of a divisional storage method (RAID0) in which data is divided for a plurality of HDDs and a duplication storage method (RAID1) in which data is duplicated and same data are stored in a plurality of HDDs. The storage device stores acquired data by using the divisional storage method or the duplication storage method in response to the data type.
Document 2 below discloses a technique which determines the most suitable transferring method for job input. An MFP (Multifunction Peripheral) disclosed in Document 2 below has a plurality of HDDs for storing data, and uses two HDDs as a single virtual storage device. The MFP determines a transferring method (RAID0 or RAID1) with respect to the two HDDs, based on the type of the job input. The MFP transfers data to each of two HDDs, based on the determined transferring method.
Document 3 below discloses an image forming apparatus which has more than two storage devices which can perform mirroring and striping. The image forming apparatus makes all the storage devices perform striping. On the other hand, the image forming apparatus makes more than one storage devices except for at least one storage device perform mirroring. When a storage device fails and the storage device was performing mirroring, the image forming apparatus replaces the failed storage device with a storage device which was not performing mirroring, as an alternative storage device. Then, the image forming apparatus continues mirroring. The image forming apparatus continues striping at all storage devices except for the failed storage device.